Aging of the immune system leads to marked alterations in the expression and formation of cytokines. The mechanisms by which these cytokine changes occur is unknown. Recently, a unique population of T- lymphocytes which bear both a natural killer (NK1.1+) and T cell phenotype (CD3+), called for this proposal NK1-T, has been demonstrated to rapidly express mRNA for a number of cytokines in mice after in vivo stimulation. Based on this, it is suggested that the NK1-T cell population helps to direct naive T cells to develop either a T eta-1 or a T eta-2 lineage. My preliminary work suggests that the NK1-T population remains constant in number during aging. Further, following in vivo stimulation with antibody to CD3, the NK1-T population shows an increase in INF-gamma mRNA expression in old versus young mice, with little change in the expression of IL-4 or IL-10. This proposal hypothesizes that the mRNA cytokine expression by NK1-T cells changes during aging. Further, it is hypothesized these changes in NK1-T cells leads to alterations in cytokine production by other lymphocytes. To assess this, the total number and phenotype of this population will first be examined in the spleen, liver, bone marrow, and thymus in young, middle and old-aged mice using flow cytometry. Second, the functional aspects of this population will be examined during aging. Specifically, mRNA expression of IL-2, IL-4, IL-10, and INF-gamma will be measured after in vivo stimulation of NK1-T cells by anti-CD3 and other methods of lymphocyte stimulation. Competitive polymerase chain reaction techniques will be used to quantitate differences between age groups. The relevance of observed changes in mRNA expression by the NK1-T cell population will then be assessed. For this, in vitro stimulation of naive CD4+T cells, with or without the presence of the NK1-T population will be done, followed by the measurement of IL-4 and INF- gamma production after re-stimulation of these CD4+ T cells. Alterations in NK1-T cells may be one of the mechanisms by which cytokine expression changes during the aging process. Understanding these mechanisms may help the treatment of altered immune responses in the aged.